ISO 11731:2017

Overview - ISO 11731:2017 (Water quality - Enumeration of Legionella)

ISO 11731:2017 specifies standardized culture methods for isolating and estimating numbers of Legionella in water. It applies to all types of water - potable, industrial, waste and natural - and to water-related matrices such as biofilms and sediments. The standard supports public‑health surveillance, outbreak investigations and verification of control measures in water systems. Note: not all Legionella species are culturable, so these methods do not recover every species.

Key topics and technical requirements

• Principle: Concentration of organisms (typically by membrane filtration), optional dilution, and transfer to selective culture media. Pre‑treatment options (heat and/or acid) reduce interfering microflora before culture.
• Selective culture: Use of media such as BCYE agar (buffered charcoal yeast extract with L‑cysteine and iron) for isolation and presumptive colony recognition.
• Confirmation: Presumptive colonies are confirmed by demonstrating growth requirement for L‑cysteine and iron(III); further species/serogroup identification requires additional tests (see Annex G).
• Sampling and processing: Guidance on sampling volumes, membrane filtration techniques (direct placing or elution/washing), concentration methods, and dilution strategies depending on expected contamination.
• Pre‑treatment options: Heat treatment and acid treatment procedures to suppress non‑target organisms and improve Legionella recovery.
• Incubation and examination: Incubation conditions (e.g., temperature control) and colony examination methods, including use of UV for enhanced visualization.
• Apparatus & quality: Requirements for microbiological equipment (incubators, membrane filters, pH meter, vortex, ultrasonic bath) and performance testing of media and methods (quality assurance, performance data).
• Documentation: Expression of results, test reporting and a decision matrix to guide method selection based on sample type and contamination level.

Applications and who uses ISO 11731:2017

• Environmental and public‑health laboratories performing routine monitoring for Legionella.
• Water system operators, building managers, HVAC and cooling‑tower maintenance teams validating control measures.
• Regulatory agencies and occupational health organizations that set compliance and surveillance programs.
• Consultants and testing service providers involved in outbreak investigations and risk assessments for legionellosis.

Practical uses include routine surveillance of hot/cold water systems, spa pools, cooling towers, dental unit waterlines, and validation of disinfection protocols.

Related standards

ISO 11731:2017 references and aligns with other standards for laboratory and sampling practice, including:

• ISO 3696 (water for laboratory use)
• ISO 7704 (membrane filter evaluation)
• ISO 8199 (general guidance on microbial enumeration by culture)
• ISO 11133 (culture media preparation and performance testing)
• ISO 19458 (microbiological sampling of water)

Keywords: ISO 11731:2017, Legionella, water quality, enumeration, culture methods, BCYE agar, membrane filtration, legionellosis, sampling, confirmation.